Application of multi-layer antistatic/antireflective coating to video display screen by sputtering

ABSTRACT

A multi-layer antistatic/antireflective coating having high electrical conductivity (10 3  ohms) and low reflectivity (0.7%) is applied to the outer surface of a video display screen by sputtering. The multi-layer coating includes an inner antistatic layer deposited directly on the video display screen and comprised of ITO, TiO 2 , etc., having a light refractive index in the range of 1.8-2.2 and a thickness in the range of 18-35 nm. The outer antireflective layer is comprised of SiO 2 , MgO, etc., having a light refractive index in the range of 1.3-1.47 and a thickness in the range of 110-140 nm. The multi-layer coating is applied using a sputtering apparatus having a dual vacuum chamber, a diffusion pump connected to one of the chambers, and plural vacuum pumps connected to the diffusion pump and to the dual vacuum chamber with various gauges and valves for monitoring and controlling the sputtering operation.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a divisional patent application of U.S.patent application Ser. No. 10/002,936 filed Nov. 15, 2001.

FIELD OF THE INVENTION

This invention relates generally to surface coatings for video displayscreens and is particularly directed to a multi-layerantistatic/antireflective coating which is applied to the video displayscreen by sputtering.

BACKGROUND OF THE INVENTION

The outer surface of a display screen, or panel, of a video displaydevice such as a cathode ray tube (CRT) is typically provided with amulti-layer coating which performs various functions. These functionsinclude reducing light transmission through the glass displayscreen/outer coating combination for improved video image contrast. Inaddition, an inner layer of the surface coating is electricallyconductive in order to shield viewers of the video display device fromlow frequency electromagnetic radiation and to dissipate electrostaticcharge on the display panel to neutral ground. The coating alsotypically provides an antireflective capability to reduce lightreflection from the display screen for ease in viewing a video image onthe display screen.

Various approaches are employed in applying the multi-layer coating tothe outer surface of a display screen. These techniques include spin andspray coating, sometimes referred to as the wet method, vacuum vapordeposition, and sputtering. Spin and spray coating methods have beenwidely used with materials containing Ag—Pd or Ag—Au colloid. While thecoating thus formed possesses good electrical conductivity andrelatively low light reflectance, it is of relatively low quality andinvolves high processing costs. These wet approaches also suffer fromproblems with reproducibility and control of the thickness of thecoating and can be used with only a limited number of solvents. Inaddition, the spin and spray coating methods have problems when usedwith materials comprised of very fine (small) particles in providinguniform particle dispersion. These approaches also suffer from thepossibility of environmental contamination.

The vacuum vapor deposition approach involves high temperature heattreatment and is thus energy intensive and more expensive than the wetcoating approach. The sputtering approach has encountered difficultiesin forming at high speed a stable SiO₂ layer having a low refractiveindex for use in the antireflective layer. One approach involvingsputtering for applying a light absorptive antireflective layer to a CRTdisplay screen is disclosed in U.S. Pat. No. 5,691,044. This approachapplies an inner layer of TiN to the surface of a glass substrate. TheTiN layer suffers from instability at the high temperatures used forapplying the multi-layer coating to the glass substrate. To improve theheat resistance of the TiN layer, an oxide barrier layer of metalnitride (TiN) is formed on the inner TiN layer. This approach requiresvarious reacting gases such as N₂ and O₂ in the sputtering process whichincreases the cost and complexity of video display screen manufacture.

The present invention avoids the limitations of the prior art byproviding a multi-layer antistatic/antireflective coating applied bysputtering to the outer surface of a video display screen which allowsfor precise control over the thickness of the multi-layer coating aswell as its light transmission and electrical resistivitycharacteristics.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to deposit amulti-layer coating on a video display screen in an environmentallyclean manner while maintaining the desired optical and electricalcharacteristics of the coating.

It is another object of the present invention to form a two-layerantistatic and antireflective coating on the surface of a video displayscreen by sputtering.

Yet another object of the present invention is to provide a low cost,highly reproducible composition for, and a method for applying to thesurface of a video display screen, a multi-layer antistaticantireflective coating having a wide range of components.

A still further object of the present invention is to provide a sputtercoating technique for depositing a multi-layer coating on the surface ofa video display screen which eliminates the need for a reactive gas andallows for close control of coating conductivity and reflectance byprecise control of individual layer thickness.

The present invention contemplates a process for forming anantistatic/antireflective coating on an outer surface of a video displayscreen comprising the steps of: sputter-depositing on the outer surfaceof the video display screen an inner metallic antistatic layer having aprecisely controlled thickness within a range of 18-35 nm, wherein alight refractive index of the inner antistatic layer is also preciselycontrolled within a range of 1.8-2.2; and sputter-depositing on theinner antistatic layer an outer antireflective layer having a preciselycontrolled thickness within a range of 110-140 nm, wherein a lightrefractive index of the outer antireflective layer is also preciselycontrolled within a range of 1.3-1.47. This invention also contemplatesa multi-layer coating for a video display panel having theaforementioned composition as well as apparatus for sputter depositing amulti-layer antistatic/antireflective coating on the surface of a videodisplay screen.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended claims set forth those novel features which characterizethe invention. However, the invention itself, as well as further objectsand advantages thereof, will best be understood by reference to thefollowing detailed description of a preferred embodiment taken inconjunction with the accompanying drawings, where like referencecharacters identify like elements throughout the various figures, inwhich:

FIG. 1 is a longitudinal sectional view of a CRT incorporating anantireflective/antistatic coating in accordance with the principles ofthe present invention;

FIG. 2 is a partial sectional view of a flat display screen having anouter surface coating comprised of an inner antistatic layer and anouter antireflective layer in accordance with the present invention; and

FIG. 3 is a simplified combined schematic and block diagram of apparatusfor applying a multi-layer antireflective/antistatic coating on theouter surface of a video display screen by sputtering in accordance withone embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a longitudinal sectional view of acolor CRT 10 incorporating an antistatic/antireflective coating 32applied by sputtering in accordance with the present invention. In thefollowing discussion the terms “display screen”, “display panel” and“faceplate” are used interchangeably. In addition, the terms “layer” and“coating” are used synonymously. CRT 10 includes a sealed glass envelope12 having a forward faceplate or display screen 14, an aft neck portion18, and an intermediate funnel portion 16. Disposed on the inner surfaceof glass display screen 14 is a phosphor screen 24 which includes pluraldiscrete phosphor deposits, or elements, which emit light when anelectron beam is incident thereon to produce a video image on thedisplay screen. Color CRT 10 includes three electron beams 22 directedonto and focused upon the CRT's glass display screen 14. Disposed in theneck portion 18 of the CRT's glass envelope 12 are plural electron guns20 typically arranged in an inline array for directing the electronbeams 22 onto the phosphor screen 24. The electron beams 22 aredeflected vertically and horizontally in unison across the phosphorscreen 24 by a magnetic deflection yoke which is not shown in the figurefor simplicity. Disposed in a spaced manner from phosphor screen 24 is ashadow mask 26 having a plurality of spaced electron beam passingapertures 26 a and a skirt portion 28 around the periphery thereof. Theshadow mask skirt portion 28 is securely attached to a shadow maskmounting fixture 30 around the periphery of the shadow mask. The shadowmask mounting fixture 30 is attached to an inner surface of the CRT'sglass envelope 12 and may include conventional attachment andpositioning structures such as a mask attachment frame and a mountingspring which also are not shown in the figure for simplicity. The shadowmask mounting fixture 30 may be attached to the inner surface of theCRT's glass envelope 12 and the shadow mask 26 may be attached to themounting fixture by conventional means such as weldments or aglass-based frit.

Referring to FIG. 2, there is shown a partial sectional view of aportion of the CRT's glass display screen 14 having the aforementionedphosphor layer 24 on the inner surface thereof and an outerantistatic/antireflective coating 32 on the outer surface thereof inaccordance with the present invention. The glass display screen 14 ofFIG. 2 is shown as being flat as the present invention is applicable toboth curved display screens as shown in FIG. 1 as well as to flatdisplay screens as shown in FIG. 2. In addition, while the presentinvention has been illustrated in the figures in terms of use on theouter surface of the display screen of a CRT, the present invention isnot limited to use with this type of display device. For example, theantistatic/antireflective coating 32 of the present invention may beused equally as well on the outer surface of the display panel ofvirtually any type of self-emitting color display device, i.e., wherethe video image is produced by phosphor activated by energetic electronsincident thereon. Self-emitting color display devices other than CRTsinclude field emission displays, plasma discharge panels, vacuumfluorescent screens, and gas discharge screens. The phosphor layer 24disposed on the inner surface of the glass display screen 14 may be inthe form of a large number of discrete dots or stripes.

In accordance with the present invention, the antistatic/antireflectivecoating 32 includes an inner antistatic layer 46 and an outerantireflective layer 48. A conductor 50 may be attached to the innerantistatic layer 46 or to the outer surface portion of the displayscreen 14 for electrically coupling the display screen to neutral groundpotential. In this manner, the build up of electrostatic charge on thedisplay screen 14 is limited by discharging the electrostatic charge onthe display screen to neutral ground via the electrically conductiveinner antistatic layer 46.

Shown in FIG. 3 is a simplified combined schematic and block diagram ofa sputter deposition apparatus 60 for applying anantistatic/antireflective coating to the outer surface of the glassdisplay screen 62 a of a CRT 62 in accordance with one aspect of thepresent invention. Sputter deposition apparatus 60 includes a dualchamber 64 comprised of a larger chamber 64 a and a smaller chamber 64 bwhich are connected together by means of a valve 65. A conventionalsputtering system 73 is disposed within the smaller vacuum chamber 64 bfor sputtering targets onto the outer surface of the display screen 62 aof CRT 62. Each of the larger chamber 64 a and the smaller chamber 64 bhas its own vacuum gauge and valve for controlling the respectivepressures therein. Thus, the larger vacuum chamber 64 a is provided withvacuum gauges 70, 74, and 84 for monitoring the pressure therein. Adischarge valve 72 allows for air to enter the larger chamber 64 a suchas for performing maintenance on the larger chamber. Vacuum gauge 66permits monitoring of the pressure in the smaller vacuum chamber 64 b,while a discharge valve 68 allows for the entry of air into the smallerchamber for inserting or removing the display screen 62 a of CRT 62. Adiffusion pump 76 is connected to the combination of the larger chamber64 a and smaller chamber 64 b via a gate 78. Vacuum gauges 80 and 82 arealso connected between the diffusion pump 76 and the combination of thelarger chamber 64 a and smaller chamber 64 b for monitoring the vacuumlevel within the diffusion pump. A pair of mechanical pumps 86 and 88are connected to the diffusion pump 76 by means of respective valves 98and 100. A vacuum gauge 94 is also connected between the mechanicalpumps 86, 88 and the diffusion pump 76 for monitoring the pressure ofthe vacuum pumps. The combination of a pair of mechanical pumps 90 and92 is coupled to the larger chamber 64 a and the smaller chamber 64 b bymeans of respective valves 108 and 106. In addition, mechanical pumps 90and 92 are coupled to the valves 106 and 108 by means of valves 102 and104, respectively, as well as by means of a vacuum gauge 96. Vacuumgauge 96 allows for monitoring the pressure of the vacuum pumps 90 and92. The use of the larger chamber 64 a in combination with the smallerchamber 64 b allows for a reduction in the pumping time required forevacuating these chambers. The combination of diffusion pump 76 andmechanical pumps 86, 88, 90 and 92 are used for evacuating the largerand smaller chambers 64 a and 64 b.

Disposed within the smaller vacuum chamber 64 b are first and secondcathodes 75 and 77. The first cathode 75 is comprised of the material tobe sputtered on the outer surface of the CRT's display screen 62 a inthe form of the first, inner antistatic layer. The second cathode 77 iscomprised of the material for forming the second, outer antireflectivelayer deposited on the inner antistatic layer. The operation of thefirst and second cathodes 75, 77 is sequential, with the first cathodeinitially actuated for depositing the inner layer, followed byactivation of the second cathode 77 for deposit of the outer layer ofthe multi-layer coating. The first and second cathodes 75, 77 aresequentially bombarded by positive ions using a positive ion source 79as is conventional in sputtering devices. As a result of this ionbombardment, the first and second cathodes 75, 77 emit extremely smallparticles of the cathode material which are deposited uniformly on theouter surface of the CRT's display screen 62 a.

The sputter deposition apparatus 60 operates in the following manner.Mechanical pumps 86 and 88 are turned on for pumping the diffusion pump76 with valves 98 and 100 in the open position. Mechanical pumps 90 and92 are turned on for pumping the larger vacuum chamber 64 a with valves102, 104 and 108 all in the open position. Valves 98, 100, 102 and 104are always open. When the pressure of the diffusion pump 76 and thepressure in the larger vacuum chamber 64 a reach the working pressure,gate 78 opens and valve 108 closes. The display screen 62 a of CRT 62 isthen loaded in an aperture 69 in the smaller vacuum chamber 64 b andvalve 106 opens for pumping the smaller vacuum chamber down to theworking pressure by means of mechanical pumps 90 and 92. A seal 71 isdisposed about aperture 69 in the smaller vacuum chamber 64 b tomaintain the smaller vacuum chamber under vacuum when the CRT 62 isinserted in the smaller vacuum chamber for coating its display screen 62a. When the pressure within the smaller vacuum chamber 64 b reaches theworking pressure, valve 65 disposed between the larger and smallervacuum chambers 64 a, 64 b opens to equalize the pressure between thetwo chambers. The sputtering system within the smaller vacuum chamber 64b then deposits the sputtering targets onto the outer surface of theCRT's display screen 62 a. This is accomplished by first bombarding thefirst cathode 75 with positive ions to produce small particles of thefirst cathode's composition which are deposited on the outer surface ofthe CRT's display screen 62 a. This first layer is the inner antistaticlayer. The second cathode 77 is then bombarded by positive ions toproduce small particles of the second cathode's composition which aredeposited in the form of a second outer layer on the inner antistaticlayer. The second outer layer is the antireflective layer of themulti-layer coating on the CRT's display screen 62 a. After coating theouter surface of the CRT's display screen 62 a with the multi-layerantistatic/antireflective coating of the present invention, valve 65closes and valve 68 opens for allowing air into the smaller vacuumchamber 64 b while the larger chamber 64 a is maintained under vacuum.The CRT 62 is then unloaded, or removed, from the smaller vacuum chamber64 b and another CRT is loaded in the smaller vacuum chamber. Forcoating the display screen of the next CRT, only the smaller chamber 64b needs to be evacuated by pumping. Once the smaller chamber 64 b isevacuated, gate 65 is opened and the pressure in the larger and smallerchambers is equal. This arrangement and procedure reduces the time forreaching the working pressure in the two chambers. The above describedsequence of steps is then repeated for the new CRT now loaded in thesmall vacuum chamber 64 b. Periodically the larger and smaller vacuumchambers 64 a, 64 b must be cleaned. Closure of gate 78 allows the twochambers to be isolated from the diffusion pump 76 while the pumpcontinues to run for cleaning the chambers. Once cleaned, the vacuumchambers are reconnected to the diffusion pump 76 for evacuation. Thisprocedure reduces downtime for maintenance and allows for the processingof a larger number of CRTs.

The sputter deposition apparatus 60 of FIG. 3 permits the thickness ofthe inner antistatic layer 46 to be controlled with great precision. Thethickness of the inner antistatic layer 46 may be controlled to within2-8 nm and is preferably in the range of 18-35 nm. The inner antistaticlayer 46 is preferably formed of a doped metallic oxide, such as indiumoxide doped with tin (ITO), tin oxide doped with fluorine (Sn0₂:F), zincoxide doped with indium (ZnO:In), zinc oxide doped with fluorine(ZnO:F), zinc oxide doped with aluminum (ZnO:Al), zinc oxide doped withtin (ZnO:Sn), or mixtures thereof. By precisely controlling thethickness of the inner antistatic layer 46, its light refractive indexmay be controlled to be within the range of 1.8-2.2. The innerantistatic layer 46 is preferably provided with a low conductivity suchas on the order of 10³ ohms and a low reflectance on the order of 0.7%.The outer antireflective layer 48 preferably includes aluminum oxide(Al₂O₃), titanium oxide (TiO₂), zinc oxide (ZnO), zirconium oxide(ZrO₂), chrome oxide (Cr₂O₃), silica (SiO₂), or mixtures thereof. Thethickness of the outer antireflective layer 48 may also be preciselycontrolled so as to be within a range of 110-140 nm. By thus controllingthe thickness of the outer antireflective layer 48, its light refractiveindex may be precisely controlled to be within the range of 1.3-1.47.The multi-layer antistatic/antireflective coating 32 preferably has highelectrical conductivity (˜10³ ohms) and low light reflectivity (˜0.7%).

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the relevant artsthat changes and modifications may be made without departing from theinvention in its broader aspects. Therefore, the aim in the appendedclaims is to cover all such changes and modifications as fall within thetrue spirit and scope of the invention. The matter set forth in theforegoing description and accompanying drawings is offered by way ofillustration only and not as a limitation. The actual scope of theinvention is intended to be defined in the following claims when viewedin their proper perspective based on the prior art.

1. In a coating consisting of two layers for a video display panel, saidlayers comprising: an inner metallic antistatic layer disposed on anouter surface of the video display panel having a precisely controlledthickness within a range of 18-35 nm, wherein a light refractive indexof said inner antistatic layer is also precisely controlled within arange of 1.8-2.2, wherein said inner metallic antistatic layer iscomprised of a doped metallic oxide comprised of Sn0₂:F, ZnO:In, ZnO:F,ZnO:Al, or ZnO:Sn, or mixtures thereof; and an outer antireflectivelayer disposed on said inner antistatic layer and having a preciselycontrolled thickness within a range of 110-140 nm, wherein a lightrefractive index of said outer antireflective layer is also preciselycontrolled within a range of 1.3-1.47, wherein said outer antireflectivelayer is comprised of Al₂O₃, TiO₂, ZnO, ZrO₂, Cr₂O₃, MgO, or SiO₂, ormixtures thereof.
 2. (canceled)
 3. (canceled)
 4. (canceled)
 5. Thecoating of claim 1 wherein said two layer coating has an electricalresistivity on the order of 10³ ohms/square.
 6. The coating of claim 1wherein said two layer coating has a reflectivity on the order of 0.7%.